In the manufacture of portland cement huge tonnages of low cost alloy grates are employed in the cooling of the cement clinker that is discharged from the rotary kilns. These grates ordinarily operate at temperatures in the region of 1200.degree. to 1400.degree. F. but may occasionally reach temperatures as high as 1500.degree. to 1600.degree. F. They are subject to corrosive attack and to surface wear by constant contact with the highly abrasive clinker.
Fesler, U.S. Pat. No. 5,106,577, discloses low cost, air meltable and castable alloys for this service. The '577 alloys have given outstanding service and have been produced by employing a wide variety of scraps and other return metallic parts and stocks. However, the '577 grate alloys include in their formulation about 0.25 to 1.5% by weight of aluminum. The inclusion of aluminum has presented manufacturing problems in the production of the cast grates because aluminum is readily oxidized at alloy melting and pouring temperatures. Further because of its low density, aluminum cannot simply be added to the molten metal during air melting in the same manner that other alloy components are handled. Therefore, in the foundry the other ingredients of the melt have been added to the furnace and brought to the molten state, but the aluminum additions have been postponed until the last. When the melt is otherwise complete, the aluminum has been added as solid lumps attached to the end of steel rods and held below the surface of the molten bath until completely melted. Alternatively, aluminum has sometimes been added in lump form to the surface of the molten metal as a ferro-aluminum, which is typically composed of about 35% Al and the balance substantially iron.
By either method, the final aluminum content in the melt is hard to control. It is possible to conduct a spectrographic analysis of the molten metal prior to pouring, then to make a further corrective addition of aluminum, if this preliminary test discloses a low content of the element. However, this procedure is not desirable because of the production delay involved and because the aluminum content may deteriorate somewhat during the holding period while an analysis is being conducted due to the additional time of contact with air. In addition, recycled scraps, sprues, gates, risers and returns of used castings from the field are widely employed in the production of cooler grates. The melting loss of aluminum from these materials is difficult to predict. Thus, aluminum recovery is inconsistent when using returns as well new materials additions during alloy preparation.
Another problem which arise from the use of aluminum is that the aluminum oxide that forms on the surface of the molten metal cannot be entirely skimmed or removed prior to pouring the castings in the foundry and this persistent oxide scum tends to be carried into the mold cavity and result in some lap and seam defects or discontinuities in solid shapes. Further, there appears to be some segregation taking place during the melting and casting procedures such that the aluminum content is often not consistent from casting to casting or, sometimes, even within an individual casting.
A potentially very severe problem in foundry production of high aluminum content alloys is that some of the frozen spillage, gates, risers or returns may accidentally be introduced into piles of materials intended for other alloys. There are many alloys in which a small amount of aluminum results in excessive brittleness. This defect has been termed "rock candy" structure due to the appearance of the coarse grain observed upon fracture of castings made from such alloys.
For all of these reasons, despite the excellent results in the field of the '577 alloys, it has remained desirable to find substitute, similar alloys, which do not include high aluminum content.
The Fesler patent also discloses the desirability of employing high carbon contents along with molybdenum to provide alloys of about 400 to 480 Brinell Hardness Number (BHN) at room temperature with resultant hot hardnesses of about 230 to 270 BHN at 1300.degree. F. For this purpose carbon contents of about 2 to 4.5%, preferably 2.8 to 4%, and molybdenum contents of about 1 to 4%, preferably 2 to 3%, have served remarkably well. Fesler also discloses a number of high silicon alloys that have proven to be far too brittle for even this low impact application.
I have now found the desirable properties of the alloys made in accordance with the '577 patent may be achieved by formulating almost the same base alloys without including any substantial amount of aluminum provided that the silicon content is greater than that employed in the Fesler alloys and falls within the range of about 2.7% to 4% by weight.